libstdc++: Fix std::common_iterator assignment [PR100823]

libstdc++: Fix std::common_iterator assignment [PR100823]

This fixes the following conformance problems reported in the PR:

- Move constructor and move assignment should be defined.
- Copy assignment from a valueless object should be allowed.

Assignment is completely rewritten by this patch, as the previous
version had a number of problems. The converting assignment failed to
handle the case of assigning a new value to a valueless object, which
should work. It only accepted lvalue arguments, so wasn't usable to
implement the move assignment operator. Finally, it enforced the
precondition that the argument is not valueless, which is correct for
the converting assignment but not for the copy assignment.

A new _M_assign member is added to handle all cases of assignment
(copying from an lvalue, moving from an rvalue, and converting from a
different type). The not valueless precondition is checked in the
converting assignment before calling _M_assign, so isn't enforced for
copy and move assignment. The new function no longer uses a switch, so
handles valueless objects as the LHS or RHS of the assignment.

libstdc++-v3/ChangeLog:

	PR libstdc++/100823
	* include/bits/stl_iterator.h (common_iterator): Define move
	constructor and move assignment operator.
	(common_iterator::_M_assign): New function implementing
	assignment.
	(common_iterator::operator=): Use _M_assign.
	(common_iterator::_S_valueless): New constant.
	* testsuite/24_iterators/common_iterator/100823.cc: New test.